Tire pressure

[Mark and Dale Bruss]

Jack +

[FastEagle]

Recommended tire inflation pressures for all vehicles built using FMVSS regulations are the sole responsibility of the vehicle manufacturer. Tire industry recommendations are only valid for inflation pressures above those recommended by vehicle manufacturers.

 

Because of the differences in construction, manufacturers of Special Trailer Tires (ST) recommend full sidewall pressures.

 

RV trailer manufacturers always have the option to set recommended inflation pressures below the sidewall pressures. However, They seldom do that. So their is zero wiggle room when vehicle manufacturers have used all of a tire's available load capacity with their recommendations.

 

Motor Home (MH) tire and rim fitments do not mirror the fitment procedures for RV trailers. They must provide a percentage of load capacity reserves - via inflation pressures - above their GAWR. Therefore their is some wiggle room for help with balancing out their loads. But, the golden rules - not to exceed GAWR, GVWR or both - are often overlooked because of the way GAWR is determined. Here again, the tire industry does not support or recommend using less inflation pressures than those recommended by the vehicle manufacturer. If you find a tire manufacturer doing that, ask them to validate their procedure.

 

FastEagle

[Jack Mayer]

Let me be very clear here.

 

There are a number - probably many - RV trailer manufacturers that put on tires that support FAR MORE weight than the GAWR and/or the GVWR on the trailer. Not all RV manufacturers use tires barely able to carry the weights.

 

In addition, MANY trailer owners upgrade their tires. Often to H rated or J rated capacities. There are valid reasons for doing this that are mentioned elsewhere. These tires are NOT ST tires, but are typically commercial tires intended for use in regional hauling or in various commercial trailer applications.

 

I want it to be clear that these tires are subject to load/inflation table guidelines - NOT MAXIMUM INFLATION. Anyone that has information otherwise I'd like to hear it, because that is not the info I get from the tire manufacturers and from their inflation guideline tables.

 

Of course, all of this has to take into account GAWR, GVWR, etc. You can't just blindly deal with the tire in a vacuum.

[Tireman9]

Could you clarify that statement some?

 

I was under the understanding that trailers running ST tires needed to run the sidewall pressure. But trailers running commercial tires like the Goodyear G114 (as an example) appropriately used the tire loading/inflation tables (otherwise, why have them?). This is directly from discussions face-to-face with a Goodyear engineer. I'd like to know if I was provided misinformation, and if the info published by Goodyear is wrong. I also don't want to confuse people any more than possible....if it is ALWAYS the case that the tire industry recommends maximum inflation for any tire on a trailer, no matter its capacity relative to current loading, then I'd like to know that and make sure that others know it. That is not my current understanding, though.

When it comes to the structural stresses in a tire's construction they are all essentially similar. It's just a question of how well thay manage and distribute the forces.

When you say you had a face to face with a tire "engineer" I would guess that he is a "Sales Engineer" rather than a "Design Engineer" Now Sales engineers know a lot of the technical stuff about tires but they do not work with Finite Element Analysis of the tire structure and work to lower the strain in the rubber compound.

 

Did you Google "Interply Shear"? Maybe you can ask your engineer how he would go about lowering this force. This is not something well known in the industry because few engineers are confronted with real life applications and how they affect tire durability.

 

We have Load inflation tables primarily for motorized vehicles i.e. vehicles where all the tires have a center of rotation that points toward the center of the turn radius. Now multi axle trailers that have 4 or 6 tires have a design that does not allow all the tires to have their center of rotation to point toward the center of the radius. This means the tires are dragged around the corner which places much higher lateral forces on the belt structure than would evern be seen on a motorized vehicle. I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

 

Now if you hade active or passive steering on trailer axles then we could look at loading & inflation differently.

[Jack Mayer]

OK, so you are saying that ALL trailers - no matter what the tire type - should be run at maximum inflation. That is a new one on me. It also means that the tires will not (necessarily) wear evenly and lower their life. And also not brake optimally because of the amount of tire on the road.

 

I'm perfectly willing to be educated on this point. But it does run counter to what other tire engineers are saying. I don't doubt the shear forces - they are always there. Including in cars and trucks. It is just a matter of "how much".

[Tireman9]

OK, so you are saying that ALL trailers - no matter what the tire type - should be run at maximum inflation. That is a new one on me. It also means that the tires will not (necessarily) wear evenly and lower their life. And also not brake optimally because of the amount of tire on the road.

 

I'm perfectly willing to be educated on this point. But it does run counter to what other tire engineers are saying. I don't doubt the shear forces - they are always there. Including in cars and trucks. It is just a matter of "how much".

Jack. I will admit that this is not something I thought about during 98% of my carreer BUT after getting involved with RV application and seeing the amount of mis-information or out of date information or just "Old Wivwes tails" going around the proverbial campfire (RV forums) I felt the need to ask some questions I did not hear others asking.

 

This idea of unique loading came to me when I watched a large 5th wheel to a 180° turn on freshly smoothed gravel. I noted tha path left by the tires was not constant radius nor was it the same for the 4 tires on the trailer. Not long after than I saw the Keystone video. At that point I asked an engineer in the Tire Mechanic dept to run a Finite Element analysis of tire structural loading of a 2 axle trailer. It turnes out that the interply shear forces on the trailer tires were some 24% higher than the shear forces on identically loaded tires on the tow vehicle. I also happened to observe a tour bus with a "Tag" axle in front of the rear most drive axle and saw it "steer"

Knowing that it was impossible for the tires to all rotate about the same center I looked at what could be done to lower the shear forces. Decreasing slip angle was one thing that would lower the force. To do this one needs to increase the tire inflation.

 

I believe that this topic is not well known or even discussed in tire industry because people have become too specialized. As an "old School" generalist I had a reputation for asking the embarising questions and for working outside my specific area of responsibility.

 

Now It is possible I am wrong. But until I can find a tire company that has the computer power to do a number of FE studies I am comfortable with my position even if I may be the only one at this point taking this position.

 

Yes higher inflation than needed to carry the load may increase tire wear but how many RV owners wear out their tires vs the number with failures or that have tires that "age-out". If my approach can eliminate a fue failures then I will be happy.

[GlennWest]

I have watched my trailer tires making a tight turn backing in. They look like they will come off the rim. It is rather unsettling. I always run my tires at max air. Mine will age out before tread wear. I believe this is true of most.

[FastEagle]

MANY trailer owners upgrade their tires. Often to H rated or J rated capacities. There are valid reasons for doing this that are mentioned elsewhere. These tires are NOT ST tires, but are typically commercial tires intended for use in regional hauling or in various commercial trailer applications.

 

I want it to be clear that these tires are subject to load/inflation table guidelines - NOT MAXIMUM INFLATION. Anyone that has information otherwise I'd like to hear it, because that is not the info I get from the tire manufacturers and from their inflation guideline tables.

 

*****************************************************************************************************************************************************

 

Part of the process for “upgrading” RV trailer tires is to use tires that are called “plus sizes”. There is a simple procedure that a well trained installer will follow. And, the use of load inflation charts for the replacement tires is a necessary step in determining new recommended tire inflation pressures for tires with different dimensions than the OE tires.

 

The bottom line of the “plus sizing” procedure is for the installer to insure that they provide equal or more load capacity than the OE tires. The new recommended tire inflation pressures act just like the recommended inflation pressures did for the OE tires. In other words, Never use less load inflation pressure than what has been recommended on the tire placard.

 

Note: Auxiliary tire placards are approved and should be placed adjacent to the original tire placard.

 

 

FastEagle

 

p.s. According to all of the various RV trailer owner's manuals I've researched an owner would need a recommendation from the trailer manufacturer to validate approved replacement tires. On the other side of the coin is; "it's mine and I'll do as I see fit". So, going from 16" tires to 17.5" tires and rims is in all probability strictly an RV trailer owner's decision.

[Tireman9]

While I agree with some of FastEagle's position i.e. The vehicle mfg has the responsibility to ensure the OE tires meet various regulatory requirements, I do not agree that here in the US owners have to follow these regulations.

 

In Europe there are such thnigs as "Homoligation" which basically requires certain replacement parts be uses and prohibits others from being uses. Here in the US we do not have similar regulations. Owners are free to make numerous modifications to their vehicles. Some may be hust cosmetic and others may be functional or even performance related.

 

Now I do not feel it is good policy for owners to make changed that lower either load capacity or lengthen stopping ability or lowering the cornering capability as these changes might result in a less safe vehicle. I do feel that if appropriate research is done and calculations made that it is possible to change tires, wheels or wheels that actually improve the capabilities of a vehicle.

 

I have not seen documents that require the vehicle manufacturer be contacted prior to making changes to a vehicle. BUT I will be contacting FastEagle and maybe I will learn something if he can provide copies of said documentation.

 

I know in my case the RV MFG (Coachmen) has not bothered to keep records for various parts on my 2008 RV so I have no idea how they would be in a position to provide recommendations concerning a change in tires. On top of that the documents they do have such as electrical diagrams have numerous errors, some of which could be safety related. I have absolutely no confidence that they have a clue about safe vehicle design and built units with the hope that they would not fail in the first 12 months, then they washed their hands of the entire thing.

 

I have written on my blog a detailed guide of the steps needed, measurements that must be recorded and calculations that need done before an owner attempts a change from one type or size tire to another. I am comfortable to stand by my guidelines at least until someone can identify a typo or math error.

[rdickinson]

2 years ago my rig was switched over to 17.5's @ 4800 lbs and 110 psi. Back when Travel Supreme was around, they told me D rated tires were fine, and had been running them for years with no problems. Well problems did occur for various reasons. The majority of the blame was levelled at under inflation in spite of owners denial. So when the time came to start on the trailer refit/upgrade, wheels and tires were first to be done. I had not heard of any problems with 17.5's so after reading the tire and wheel topic, they were ordered and installed.

Having said that, TS isn't around and are not able to provide tire pressure on 17.5's.

My trailer is about 22000 lbs which is over the 19900 on the sticker. Not sure about pin wt. Pin wt will not be the same as landing gear weight. I could weigh both sides of the trailer by turning around and face the other way with one set of axles on and the other off the scales.

 

In BC we have a number of scales which have been scrapped, shack and staff are gone but the scales still operate.

 

So I am not sure where that puts me trailer tire pressure wise.

 

Around mid June I may be heading to Salt Lake City to get Motosat up and running again then out to Michigan and surrounding areas to some tractor pulls. Who can I see or where can I go, enroute, to have the rig weighed and get this sorted out once and for all?

[Jack Mayer]

Roger any truck scale can give you a pretty good weight and you can calculate the pin weight. At the HDT Rally Trey and Susan do individual wheel weights, which is the best info you can get. As you know, our trailers tend to be uneven in their loading.

 

Given you cannot get individual wheel weights easily, what I do is to calculate the tire load based on even distribution of the ENTIRE weight of the trailer, including pin weight. Obviously, the pin weight is not on the trailer most of the time when towing. But it does transition partially to the tires at points in time. Given I don't know individual wheel weights I use the total as a substitute....that gives me a wide margin of safety for any individual wheel position that is "heavy". And also gives me a safety margin for load shifting based on road surface variation....you can see that in effect going over speed bumps. IMO, that is the best you can do without individual wheel weights.

 

Lets take my 2015 New Horizons as an example. The total trailer weight loaded is 28,200 lbs. That includes the pin weight. The pin is 7,000 leaving the axles at 21,200 lbs. But the axles are not evenly loaded so you would want at LEAST a 10% safety margin, which is 23320 or 7067/axle; 3533/tire. Using the Goodyear tire charts for the G114 tires that would put you at the 90psi level - chart says 3695 lbs. That gives you more than 10% safety margin, but I would still bump up to the next chart position and run 95 psi. The chart says 3860 lbs per tire for that psi.

 

Instead of doing it this way, I use the total trailer weight instead. So using the weight of the entire trailer of 28,200, the per axle is 9400; the per tire is 4700. The chart says that for this tire weight I need between 120 and 125 psi. So I would run 120 (lots of safety factor there).

 

You get to choose if you want to support the entire trailer or just somewhat more than your axle weights.

 

Tireman's position would be to run 125 psi all the time - no matter what the weights.

[Mark and Dale Bruss]

Great answer Jack

[Tireman9]

2 years ago my rig was switched over to 17.5's @ 4800 lbs and 110 psi. Back when Travel Supreme was around, they told me D rated tires were fine, and had been running them for years with no problems. Well problems did occur for various reasons. The majority of the blame was levelled at under inflation in spite of owners denial. So when the time came to start on the trailer refit/upgrade, wheels and tires were first to be done. I had not heard of any problems with 17.5's so after reading the tire and wheel topic, they were ordered and installed.

Having said that, TS isn't around and are not able to provide tire pressure on 17.5's.

My trailer is about 22000 lbs which is over the 19900 on the sticker. Not sure about pin wt. Pin wt will not be the same as landing gear weight. I could weigh both sides of the trailer by turning around and face the other way with one set of axles on and the other off the scales.

 

In BC we have a number of scales which have been scrapped, shack and staff are gone but the scales still operate.

 

So I am not sure where that puts me trailer tire pressure wise.

 

Around mid June I may be heading to Salt Lake City to get Motosat up and running again then out to Michigan and surrounding areas to some tractor pulls. Who can I see or where can I go, enroute, to have the rig weighed and get this sorted out once and for all?

 

 

If you can provide that complete tire size I can look up the Load Infl info. If you can also provide the actual load on the heavier side for each axle I can advise the Minimum CIP.

 

If you only have the Total load on each axle then I would use the conservative load split of 45/55% until I had data showing otherwise.

[rdickinson]

Tire size I can get. I may even be able to narrow down the wheel weights by putting 1-2-3 wheels at a time on the scales and do that for both sides. That will have to wait as truck is of for more modifications and moving the trailer is a pain due to tight quarters.

 

Mid June is when I expect to be on the road, Oregon, Salt lake City maybe then out to Tractor pulls, Michigan Illinois etc for a couple of weeks.

 

I could divert to a location able to weigh the rig then.

 

I will put it on tonight " on edit"

[FastEagle]

People reading my tire posts often misunderstand my dialog as being binding on them. My dialog is about how to do things. How to find where it tells how to do things. My dialog is about safety. My dialog is about options.

 

To make interpretations of regulations or standards, researching them for the true meanings can be difficult. Many interpretations are self descriptive, others are ambiguous.

 

Regulations for RV trailer manufacturers to follow (FMVSS) when certifying their product are not binding on the public at large. On the other hand, they are not for the public at large to apply.

 

NHTSA is tasked with writing and enforcing vehicle safety. They can impose their will upon the vehicle manufacturers but generally not on the public at large. That’s mostly the responsibility of each individual state.

 

The TRA provides the tire industry with many of their safety standards. Here again the standards my not be binding on the public but you can bet those tort people will use any unsafe practice as a reason for action.

 

SAFECAR is one of the mediums used by NHTSA to get the word out to the people. Here is one of those PDFs to ponder.

 

http://www.safercar.gov/tires/pages/tires_maintenance.html

 

I go to a lot of RV shows. I take pictures and verify things. Here is an example of one of my pictures. It’s information is accurate and correct.

 

http://www.irv2.com/photopost/showfull.php?photo=20840

 

In the FMVSS regulations the vehicle manufacturer is directed to select tires that are appropriate for fitment to a trailer’s GAWR (s). They are also directed to set the recommended - correct - cold inflation pressures for those tires. They are also directed to put that information on the vehicle’s certification label and affix it to the left forward external portion of the trailer. I personally interpret that information to be the minimum tire size and inflation pressure for those Original Equipment tires.

 

 

FastEagle

[jadatis]

When it comes to the structural stresses in a tire's construction they are all essentially similar. It's just a question of how well thay manage and distribute the forces.

When you say you had a face to face with a tire "engineer" I would guess that he is a "Sales Engineer" rather than a "Design Engineer" Now Sales engineers know a lot of the technical stuff about tires but they do not work with Finite Element Analysis of the tire structure and work to lower the strain in the rubber compound.

 

Did you Google "Interply Shear"? Maybe you can ask your engineer how he would go about lowering this force. This is not something well known in the industry because few engineers are confronted with real life applications and how they affect tire durability.

 

We have Load inflation tables primarily for motorized vehicles i.e. vehicles where all the tires have a center of rotation that points toward the center of the turn radius. Now multi axle trailers that have 4 or 6 tires have a design that does not allow all the tires to have their center of rotation to point toward the center of the radius. This means the tires are dragged around the corner which places much higher lateral forces on the belt structure than would evern be seen on a motorized vehicle. I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

 

Now if you hade active or passive steering on trailer axles then we could look at loading & inflation differently.

 

My believe is that if the stucture of the rubber is intact , the sheer forces are no porblem.

It gets different if by heat , produced by driving at a sertain speed. When sertain rings of the tire get to hot , the rubber gets hardened and will damage in next bendings of rubber .

 

ST tires are calculated in their maximum load for more deflection allowed because of the lower maximum speed used of 65m/h.

But its always better to give the ST tire lesser deflection so lesser heatproduction , and lesser fuel needed . Best is to give an ST tire the same deflection an LT tire is calculated for . Even if the pressure needed for the load to give that deflection is up to 10 psi above the AT-pressure ( TS 110 psi), and most tire-makers still allow that . For truck tires even 20 psi higher allowed .

 

There is a large range in deflection, so pressure for a load, or load for a pressure, wihtin the tire stays with its width on the ground so even wear.

This gives for ST tires that if you use AT-pressure ( wich is not the maximum pressure by the way) you can support maximum load and no damage to tire at 65 m/h. When real load goes down to about 60 % of maximum load still not to hard so bumping happens.

Thats the reason why AT-pressure ( wich most think is maximum pressure) is adviced for TT and 5thW, because mostly the loads are close to the maximum load of tire.

Best is to give the tire a pressure for the load ( so deflection) wich would give yust no bumping on the highest loaded tire on the axle.

Then the pressure can drop in time, or load can be more incidentially or misyudging of load to low, or unequall load R/L , or speed higher , with still no damage to tire. And because you are at the upper side of the range, still no discomfort for MH or screwsloose for T.

Bycomming advantage is lesser fuel consumption.

 

Multi axle TT or 5thwh more get damaged by alignment problems by hitting pavement, a tire that rotates under an angle on the road gets hotter so sooner damaged.

 

And no I am not working in the tire busines, but got wiser in time by googling after I got hold of the European official used formula to calculate pressure for load and went running with it.

Call myself tire-pressure specialist nowadays.

[Tireman9]

 

My believe is that if the stucture of the rubber is intact , the sheer forces are no porblem.

It gets different if by heat , produced by driving at a sertain speed. When sertain rings of the tire get to hot , the rubber gets hardened and will damage in next bendings of rubber .

 

ST tires are calculated in their maximum load for more deflection allowed because of the lower maximum speed used of 65m/h.

But its always better to give the ST tire lesser deflection so lesser heatproduction , and lesser fuel needed . Best is to give an ST tire the same deflection an LT tire is calculated for . Even if the pressure needed for the load to give that deflection is up to 10 psi above the AT-pressure ( TS 110 psi), and most tire-makers still allow that . For truck tires even 20 psi higher allowed .

 

There is a large range in deflection, so pressure for a load, or load for a pressure, wihtin the tire stays with its width on the ground so even wear.

This gives for ST tires that if you use AT-pressure ( wich is not the maximum pressure by the way) you can support maximum load and no damage to tire at 65 m/h. When real load goes down to about 60 % of maximum load still not to hard so bumping happens.

Thats the reason why AT-pressure ( wich most think is maximum pressure) is adviced for TT and 5thW, because mostly the loads are close to the maximum load of tire.

Best is to give the tire a pressure for the load ( so deflection) wich would give yust no bumping on the highest loaded tire on the axle.

Then the pressure can drop in time, or load can be more incidentially or misyudging of load to low, or unequall load R/L , or speed higher , with still no damage to tire. And because you are at the upper side of the range, still no discomfort for MH or screwsloose for T.

Bycomming advantage is lesser fuel consumption.

 

Multi axle TT or 5thwh more get damaged by alignment problems by hitting pavement, a tire that rotates under an angle on the road gets hotter so sooner damaged.

 

And no I am not working in the tire busines, but got wiser in time by googling after I got hold of the European official used formula to calculate pressure for load and went running with it.

Call myself tire-pressure specialist nowadays.

Hi Jadatis

You said "ST tires are calculated in their maximum load for more deflection allowed because of the lower maximum speed used of 65m/h". That may be true but based on the relatively high rate of belt separations (much higher than with LT or P type tires in their normal application) I believe that ignoring the extremely high lateral loads seen with multi-axle trailers, the science used in the '50s and '60s when ST tires were only used on single axle trailers and when the maximum highway speed was 55 mph and most trailers were towed at 45 to 50 mph, things were different.

The real life use of tires on trailers is much different today. Speeds are not 45 to 55 but are 60 to 75 mph in many applications so this generates too much heat so the rubber heats up and is degraded many times faster.

(Remember the rate of degradation doubles with each increase in heat of 20°F)

In ADDITION the move from single axle to multi axle use has also increased the Interply Shear by well over 20%.

 

IMO the increase in shear forces along with the faster degradation rate of belt rubber has reduced tire life by half or more of what it might have been. The application features (speed, load and Interply Shear) have moved the market features away from what the original design intent was so the Science of the 60's simply does not provide sufficient margin for the use these tires see today.

 

 

You said "deflection is up to 10 psi above the AT-pressure ( TS 110 psi), and most tire-makers still allow" Sorry but "most tire-makers" have NOT issues any technical bulletins that encourage or allow a 10 psi increase. I have only seen one such notice and it is not reasonable to apply a notice from one company on the products from another. I suggest you do more "Googling"